1. Field of the Invention
The present invention concerns a method for automatic microphone signal equalization or balancing or adjustment in a directional microphone system with at least three omnidirectional microphones, wherein the two omnidirectional microphones are electrically connected in respective pairs to form first and second directional microphones of the first order to generate a directional characteristic.
The present invention also concerns a directional microphone system with at least first, second and third omnidirectional microphones, wherein the first and the second omnidirectional microphones are electrically connected with one another to form a first directional microphone of the first order, and the second and the third omnidirectional microphones are electrically connected with one another to form a second directional microphone of the first order.
2. Description of the Prior Art
Hearing impaired persons frequently suffer a reduced communication capability in the presence of interfering noise. To improve the signal-to-noise ratio, directional microphone arrangements have been used for some time, the benefit of which is indisputable for hearing impaired persons. Frequently, either a system of the first order (meaning with two microphones) or a system of a higher order is used. The exclusion of noise signals received from behind, as well as the focusing on frontally incident sounds, enables a better comprehension in everyday situations.
A hearing device with three omnidirectional microphones is known from PCT Application WO 00/76268. One directional microphone of the first order is formed from two microphones by the inversion and delay of the microphone signal generated by one of the microphones and the subsequent addition of both microphone signals. A directional microphone with a directional characteristic of the second order (directional microphone of the second order) likewise can be formed by the delay and inversion of the microphone signal formed by a directional microphone of the first order and the subsequent addition to a microphone signal formed by a directional microphone of the first order.
Particularly in the case of directional microphones of higher order, the problem occurs that the systems are extremely sensitive with regard to detunings of the transfer function of the microphones according to magnitude and phase that, for example, are caused by aging and contamination effects. While often an amplitude tuning of the microphones is sufficient given the use of directional microphones of the first order in hearing devices, the phase relation of the individual microphones to each other must also be very precisely tuned in the case of directional microphones of higher order.
A hearing device with automatic microphone adjustment, as well as a method for operation of such a hearing device, is known from German OS 198 22 021. In this known hearing device, a difference element is provided for subtraction of average values of the output signals of the microphones, and an analysis/control unit is connected subsequent to (downstream from) the difference element to regulate the amplification of the output signal of at least one microphone. The regulation of the amplification ensues such that the average values of the microphone signals are brought into agreement. Only the amplitudes of the microphones are adjusted in this known microphone equalization.
A hearing aid device with a directional characteristic is known from German PS 199 18 883. In this hearing aid device, high-pass filtering connected subsequent to the microphones are adapted with regard to their lower limit frequency for amplitude and/or phase adjustment of two omnidirectional microphones. The lower limit frequency of one microphone is compensated by a high-pass filter (downstream from the microphone) at the limit frequency of the other microphone.
A hearing device as well as a method for equalizing the microphones of a directional microphone system in a hearing device are known from German OS 198 49 739. In a directional microphone system with at least two microphones, in order to prevent an undesired falsification of the directional microphone characteristic due to the microphones not being tuned to one another, characteristic values of the signals of both microphones are detected by a equalization element, a control element and an adjusting element and are compensated to one another given a detected deviation.
A disadvantage of the known methods for microphone equalization in directional microphones is they have an insufficient effect given incorrect tuning of the microphones that in particular is caused by aging and contamination effects.